The invention relates to an electric power-assisted steering system making it possible to apply an assist torque for the angular displacement of the wheels of a vehicle, said torque being supplied by an electronically commutated motor comprising P phases and N pairs of poles.
The invention is particularly applicable to conventional steering systems comprising a steering column associated mechanically with the wheels by means of a rack pinion and a rack, so as to convert the rotational movement of the steering wheel associated with the column into angular displacement of the wheels.
The invention is also applicable to decoupled steering systems in which there is no direct mechanical link between the steering wheel and the wheels, whether the vehicle is real or belongs to a simulation system.
This simulation can be the subject of a game, connected with training in a driving school, or connected with an interactive driving simulation for the requirements of motor vehicle manufacturers.
On account of their high specific power, electronically commutated motors, or brushless motors, are particularly adapted to be used in an electric power-assisted steering system.
Within the scope of this application, it is necessary to control precisely the commutation of the current in the phases of these motors in order to obtain a torque which is optimum and equal to a set point with the fewest modulations possible.
To do this, use is known of a sensor comprising three sensing elements disposed opposite an encoder having N pairs of poles, said encoder being rotated by the motor.
In the case of a three-phase motor, by providing that the three sensing elements are phase-displaced with respect to one another by a mechanical angle making it possible to supply three electrical signals phase-displaced by 120 electrical degrees, it is possible to control the commutation between the phases of the motor at the right moment.
But this type of implementation can cause modulations of the motor torque which are due in particular to:
the reluctance torque of the motor;
faults of commutation of the current from one phase to another depending on the speed of rotation of the motor;
the absence of overlapping of the trapezoidal EMFs.
There is known, in particular from the document FR-2 749 452, a device for controlling an electronically commutated electric motor which makes it possible to partially avoid the torque faults related to the commutation of the current from one phase to another.
But, in order to avoid the torque modulation related to the motor design (reluctance torque and absence of overlapping of the trapezoidal EMFs), it then becomes necessary to know to a fine degree the absolute position of the rotor in order to control the current in the phases according to the position of the rotor.
Such a control strategy therefore necessitates an additional sensor giving a high-resolution position in addition to the one controlling the commutation.
This type of solution, in providing two sensors, leads to significant integration constraints and a non-optimised cost. Moreover, it can be penalising in terms of mechanical inertia of the rotor.
The invention aims in particular to resolve these drawbacks by proposing an electric power-assisted steering system comprising a two-track encoder movable rotation-wise in front of a single sensor fixed rotation-wise, said sensor being capable, after appropriate electronic processing, of delivering on the one hand signals for commutating the current in the phases and on the other hand high-resolution position signals for allowing the control of the current supplying the phases, with a sufficiently fine resolution to avoid motor torque modulations.
Moreover, the invention is applicable to electric power-assisted steering systems without resulting in significant integration constraints.
To that end, the invention proposes an electric power-assisted steering system making it possible to apply an assist torque for the angular displacement of the wheels of a vehicle, said torque being applied by an electronically commutated motor comprising P phases and N pairs of poles, said system also comprising a device for controlling the motor comprising:
a circular multipole encoder intended to perform, together with the rotor of the motor, a plurality of revolutions around its axis of rotation, said encoder comprising a main multipole track and a multipole commutation track comprising magnetic singularities equidistributed angularly, the main and commutation tracks being concentric on the encoder;
a fixed sensor disposed opposite and at air gap distance from the encoder, said sensor comprising at least three sensing elements, at least two of which are positioned opposite the main track so as to deliver square digital signals A, B in quadrature, and at least one of which is positioned opposite the commutation track 30 as to deliver a signal C in the form of 2*P*N pulses per revolution of the rotor;
a circuit for commutating the currents in the phase windings of the motor having 2*P*N switches;
a control circuit for the commutation circuit which:
from the signal C supplies commutation signals for the switches; and
from the signals A, B and according to a torque set point defines a current set point which controls the current supplying the phase windings of the motor.